What size water softener do you need?

What size water softener do I need?

Multiply the people in the house by the gallons each uses a day and by the water hardness in grains per gallon; that is the hardness removed each day. Multiply by the days between regenerations for the capacity you need. Example: 4 people × 82 gallons × 10 gpg = 3,280 grains a day; regenerating every 7 days takes 22,960 grains of capacity.

How much water does a person use a day?

EPA WaterSense says each American uses an average of 82 gallons of water a day at home — indoors and out. The Water Quality Association suggests sizing a softener for a typical family of four using about 200 gallons a day, 50 each, warns against oversizing and recommends bypassing outdoor taps so only indoor water is softened. Your water bill gives your own number.

How do I convert ppm or mg/L to grains per gallon?

Divide by 17.1. A grain is 64.79891 milligrams and a U.S. gallon 3.785411784 liters, so 1 grain per gallon is 17.12 mg/L (ppm) of hardness as calcium carbonate. A test result of 180 mg/L is 10.5 gpg.

How hard is my water?

The U.S. Geological Survey classifies 0 to 60 mg/L as calcium carbonate as soft, 61 to 120 mg/L as moderately hard, 121 to 180 mg/L as hard and more than 180 mg/L as very hard. In grains per gallon those limits are about 3.5, 7 and 10.5.

How much salt will the softener use?

The WQA's best practice is to set softeners to remove 4,000 grains of hardness per pound of salt. At that setting 22,960 grains take about 5.7 lb of salt per regeneration. Your softener's manual gives its own figure. The WQA also notes that iron above 2 ppm or dissolved solids above 750 ppm keep high-efficiency softeners from reaching that setting, and that above 30 gpg some hardness may leak through.

Does regenerating less often save salt?

Not by itself. The softener has to remove the same grains of hardness over a year, and at a fixed setting of 4,000 grains per pound of salt that costs the same salt however the grains are grouped: 274 lb a year for 3,000 grains a day. A larger unit regenerating every 10 days uses 7.50 lb per cycle against 3.75 lb every 5 days, 37 cycles against 73. What saves salt is a more efficient setting (more grains removed per pound) and softening only the water that needs it.

What is the difference between grains per gallon and mg/L?

They are two ways to express the same hardness. One grain per gallon is 17.1 mg/L (ppm) as calcium carbonate, so water at 15 gpg measures about 257 mg/L, which the USGS classes as very hard. Water-test reports from labs usually give mg/L, and softener capacity is quoted in grains.

How the calculation works

The softener has to take out the hardness in all the water you soften. Daily water use (people times gallons per person) times the hardness in grains per gallon gives the grains to be removed each day. Multiplying by the number of days between regenerations gives the capacity of resin you need, in grains. The salt for each regeneration is that capacity divided by the grains removed per pound of salt, which is a setting of the softener; the Water Quality Association's practice is 4,000 grains per pound. The calculator multiplies by 365 divided by the days to give salt per year. Iron, manganese and other dissolved substances are not part of this calculation.

Worked example: four people, 15 gpg

At the WQA's figure of 50 gallons per person per day, four people use 200 gallons a day. At 15 grains per gallon that is 3,000 grains a day. If the softener regenerates every 5 days, it needs 15,000 grains of capacity, and at 4,000 grains per pound it uses 3.75 lb of salt per regeneration, 274 lb a year. A 15 gpg water is 257 mg/L, in the very hard class of the U.S. Geological Survey. Because capacity is sold in round numbers, you pick the next size up, but the WQA warns against a unit much larger than needed.

Outdoor taps and special cases

Water used outdoors, for lawns and car washing, does not need softening and adds nothing to the household need; the WQA recommends bypassing outside taps so that only indoor water goes through the softener. Hard water with iron above 2 ppm, dissolved solids above 750 ppm, or hardness above 30 gpg can keep a high-efficiency softener from working at its best setting, and may need pretreatment. A test of your own water, from the supplier's report or from a lab, is the right basis for the hardness number. Wells vary from season to season.

Common mistakes

Using the nationwide average of 82 gallons per person, which includes outdoor use, to size an indoor softener inflates the need. Entering mg/L into the gpg box (or the reverse) is off by a factor of 17. Forgetting to test the water leads to guessing the hardness. Over-sizing is a mistake too: the Water Quality Association warns against a unit much larger than the household needs. And a softener removes hardness, not bacteria, iron staining at high levels or contaminants such as lead; those need other treatment.

Sources: U.S. EPA WaterSense, statistics and facts on water use (82 gallons per person per day, from USGS estimates); Water Quality Association, "Getting Smart with Softeners" (50 gallons per person for a family of four, 4,000 grains per pound of salt, bypassing outdoor taps, limits for iron, dissolved solids and hardness); U.S. Geological Survey Water Science School, "Hardness of Water"; NIST Handbook 44, Appendix C for the grain and the gallon. Related: water heater calculator, and water intake calculator.

Sources: U.S. EPA WaterSense, “Statistics and Facts”; U.S. Geological Survey Water Science School, “Hardness of Water”; Water Quality Association, “Getting Smart with Softeners”; NIST Handbook 44, Appendix C (grain and gallon). How we check our calculators.